Leeuwenhoeck observed, in the semen of a cock, animals which resemble the figure of an eel, but so exceedingly minute, that he pretends fifty thousand would not equal in size a grain of sand; and in that of a rat many millions would be required to make the thickness of a hair, &c. This observer imagined that the whole substance of the semen was only a mass of these animalcules. He perceived these animalculæ in the semen of men, quadrupeds, birds, fishes, insects, &c. In that of grasshoppers they were long and slender. They are attached, he says, by their extremities, and the inferior of which he calls the tail, had a quick motion, like that of the tail of a serpent, when the upper part is motionless. He further adds, that in the semen of young animals the animalculæ are motionless, but as the age for reproduction comes on they move about with great vivacity. In the semen of a male frog he observed animalculæ, at first they were imperfect and motionless, but some time afterwards he found them living: they were so very small, he says, that ten thousand would scarcely equal the size of a single egg of the female. It was only those in the seminal liquor of the frog which had life and motion. In the semen of a man, and that of a dog, he pretends to have seen two kinds, which he looked upon as males and females. Having inclosed the seed of a dog in a vial, he says, that numbers of the animalculæ died the first day; the second and third there died still more, and very few remained alive the fourth. But having repeated this experiment on the semen of the same dog, he found, at the end of seven days, live animalculæ, some of which swam with as much swiftness as in fresh-extracted semen; and having opened a bitch which had been three times with the same dog, he could not perceive by the naked eye any seminal liquor of the male in either of the horns of the matrix; but by help of a microscope he discovered the spermatic animals of the dog in both horns of the matrix, and great numbers of them in that part of the matrix adjoining to the vagina, which, says he, evidently proves that the male semen enters the matrix, or at least that the spermatic animals of the dog had got there by their own motion, which is sufficient to carry them four or five inches in half an hour. In the matrix of a doe rabbit, which had just received the buck, he likewise observed an infinite number of spermatic animals; he says, that their bodies are round, with long tails, and that they often change their forms, especially when the humid matter in which they swim evaporates and dries.